Conventionally, a ball screw device in which a screw shaft and a nut member are threadedly engaged with each other through the intermediation of a number of balls is well known. In such a ball screw device, a spiral ball rolling groove formed in an outer peripheral surface of the screw shaft and a spiral load rolling groove formed in an inner peripheral surface of the nut member face each other to form a load passage, with balls effecting load application between the screw shaft and the nut member while rolling in this load passage. Further, the nut member is equipped with a non-load passage for circulating balls having rolled through the load passage back to the load passage. As the screw shaft and the nut member make relative rotation, the balls endlessly circulate from the load passage to the non-load passage, and from the non-load passage to the load passage.
Ball screw devices can be classified into several types according to the structure of the non-load passage, including a so-called end cap type ball screw device. Examples of the end cap type ball screw device are disclosed in JP 06-201013 A and WO97/48922, the former of which is shown in FIG. 16. In this ball screw device, a nut member 100 is composed of a nut main body 102 in which a load rolling groove 101 as mentioned above is formed and a pair of end caps 103 attached to the axial end surfaces of the nut main body 102. More specifically, a ball return hole 104 parallel to the axial direction is formed in the nut main body 102, and the end caps 103 attached to the end surfaces of the nut main body 102 have scoop portions 108 for dislodging balls 107 from a ball rolling groove 106 of a screw shaft 105 and direction switching passages 109 for guiding the dislodged balls 107 to an entrance of the ball return hole 104. By fixing the end caps 103 to the nut main body 102, the ball return hole 104 and the direction switching passages 109 are communicatingly connected together to complete a non-load passage for the balls 107. In addition, the non-load passage of this end cap type device circulates the balls from one to the other end of the nut main body, so that the device has adaptability to an increase in the total length of the nut main body, which is advantageous when the lead of the ball rolling groove formed in the screw shaft is fast. Thus, this end cap type ball screw device is most suitable for linearly guiding a movable body, such as a table, at high speed.
However, as described above, in the conventional end cap type ball screw device, the end caps have scoop portions for dislodging the balls from the ball rolling groove of the screw shaft and direction switching passages for guiding the dislodged balls to the ball return hole, so that the thickness of the end caps must be rather large. Thus, when the end caps are fixed to the end surfaces of the nut main body, the total length of the nut member is rather large for the magnitude of the load applied, thus making it impossible to achieve a reduction in the size of the nut member.
To reduce the thickness of the end caps, it is expedient if the direction switching passages 109 in the end cap 103 extend in a direction perpendicular to the axial direction of the screw shaft 105 as in the case of the conventional ball screw device shown in FIG. 16. However, the ball rolling groove 106 of the screw shaft 105 is formed spirally, and the balls 107 rolling in this ball rolling groove 106 also have a speed component in the axial direction of the screw shaft 105. Thus, when the direction switching passages 109 extend in the above-mentioned direction, the advancing direction of the balls 107 dislodged from the ball rolling groove 106 is changed forcibly, with the result that the balls 107 repeatedly collide with the end caps 103. Thus, when the balls 107 are circulated at high speed, there is a fear of the end caps 103 being damaged. Further, a great resistance is offered to the circulation of the balls 107, which leads to a limitation in the high speed feeding of the movable body. Further, the balls 107 and the end caps 103 generate collision sound, which leads to a problem of noise during use.